System and method for delivery of variable oxygen flow

ABSTRACT

A method and apparatus to deliver a variable flow of oxygen to a patient is described. The apparatus may include a flow control valve, a pressure sensor to detect a patient&#39;s breathing pressure and ambient pressure, an oxygen flow analyzer to measure oxygen flow to the patient, and a processor to analyze the breathing pressure values, ambient pressure value, and oxygen flow rate values and to determine when a patient is inhaling. When the processor determines the patient is inhaling, the processor calculates an optimal oxygen flow rate to deliver to a patient, which may depend on a pre-selected flow rate and an oxygen backlog, and the processor sends a signal to the flow control valve to deliver the optimal oxygen flow rate to the patient.

FIELD OF INVENTION

This invention relates generally to breathing detection systems andmethods and oxygen delivery systems and methods. It is particularlydirected to a system and method for delivery a variable flow of oxygento a patient.

BACKGROUND

Inadequate ventilation may be one cause of hazards during proceduralsedation. Apnea, or lack of respiration, can be caused by the drugs thatthe patient receives through the intravenous port during sedation.Central apnea occurs when the patient makes no effort or little effortto breathe and is often caused by giving too much opioid agent to thepatient. Obstructive apnea occurs when the airway tissues are so relaxedthat they block gas flow, thereby reducing the volume of air that thepatient receives. Obstructive apnea during sedation is often caused bygiving too much sedative agent to the patient. Respiration is usuallymonitored during sedation using a pulse oximeter that measures theoxygen saturation in the patient's arterial blood. Respiration rate isusually monitored during sedation using transthoracic bioimpedance orcapnometry. Transthoracic bioimpedance measures the change in electricalimpedance across the chest using the EKG electrodes, and is oftenunreliable. Additionally, transthoracic bioimpedance cannot detectairway obstruction since the patient's chest still moves during anattempt to force gas through an obstructed airway.

Capnometry measures the concentration of carbon dioxide in the exhaledair and calculates a breath rate by measuring the time betweenexcursions in the carbon dioxide signal. Capnometry is expensive and isknown in the field to detect breath attempts as true breaths. Becausecapnometry does not measure the total volume of exhaled carbon dioxide,only a small volume of exhaled carbon dioxide is needed to cause adetection of a true breath. An additional limitation of capnometry isthat the signal is degraded when high oxygen flow is given. This is achallenge because when a patient is hypoxic, the oxygen flow is oftenincreased; however increased oxygen flow dilutes the exhaled carbondioxide, thereby diminishing the clinician's ability to monitorrespiration in the patients that have a great need to be monitored.

Additionally, there is a clinical controversy regarding the use ofoxygen during sedation. While some believe oxygen is beneficial, othershave stated that addition of oxygen is detrimental because it allows useof excess opioid and sedatives and that added oxygen only delaysdetection of inadequate respiration by the pulse oximeter.

While oxygen is delivered clinically at fixed flow rate that runscontinuously, most of the oxygen delivered is not actually inhaled bythe patient. The majority of the supplemental oxygen is wasted as thepatient is breathing out (exhaling). Furthermore, even when the patientis inhaling, there is only a brief time when the patient is inhalingwith sufficient rate to take in all of the oxygen that is beingdelivered through the nasal cannula or mask. It may be helpfulclinically to calculate the fraction of the supplemental oxygen thatactually enters the patient's airways.

Consequently, there is a long felt need for a technology that could moreaccurately detect a patient's breathing, especially in patientsreceiving opioids and sedatives. This technology would desirably furtherdifferentiate between a true apnea and mere displacement of an oxygendelivery device. It may also be desirable to be able to determine howmuch of the oxygen delivered to the patient is being inhaled by thepatient.

SUMMARY OF THE INVENTION

This present disclosure relates to methods and systems for identifyingthe source of oxygen delivery failure to a patient. In one aspect, themethod may comprise:

detecting a patient's breathing pressure value to determine a pluralityof breathing pressure values;

measuring an oxygen flow rate through an oxygen delivery device toprovide an oxygen flow rate value;

measuring an ambient pressure to provide an ambient pressure value;

receiving the plurality of breathing pressure values, the oxygen flowrate value, and the ambient pressure value at at least one processor;

the at least one processor analyzing the plurality of breathing pressurevalues to determine if a patient is breathing or not breathing, and whenthe processor determines the patient is not breathing and the oxygenflow rate value is greater than a predetermined threshold value, theprocessor further comparing the plurality of breathing pressure valuesto the ambient pressure value; and

wherein the processor outputs an apnea alarm when the plurality ofbreathing pressure values is greater than the ambient pressure value,and wherein the processor outputs an oxygen delivery device displacementalarm when the plurality of breathing pressure values is equal to theambient pressure value.

According to one aspect, the step of measuring an oxygen flow ratethrough an oxygen delivery device comprises measuring the oxygen flowrate with a differential-type pressure analyzer. According to adifferent configuration, the step of measuring an oxygen flow ratethrough an oxygen delivery device comprises measuring the oxygen flowrate with a heated wire, or heated film type anemometer.

According to another aspect, a pressure sensing device with a solenoidvalve may perform the steps of detecting a patient's breathing pressureand measuring an ambient pressure are performed by a pressure sensingdevice provided with a solenoid valve. The step of measuring an ambientpressure may comprise opening the solenoid valve on the pressure sensingdevice to ambient pressure.

In some aspects, the processor may output an audible alarm for the apneaalarm and the oxygen delivery device displacement alarm. In anotherconfiguration, the processor may output a visual alarm for the apneaalarm and the oxygen delivery device displacement alarm. In oneconfiguration, the predetermined threshold of the oxygen flow rate maybe 0.5 liters per minute.

According to some aspects, the step of detecting a patient's breathingpressure value to determine a plurality of breathing pressure valuescomprises detecting a patient's breathing pressure value for apredetermined amount of time. In another aspect, the method comprisesmedical personnel selecting a predetermined amount of time.

According to another aspect, the at least one processor comprises anoxygen flow analyzer processor, and a second processor, the oxygen flowanalyzer processor in communication with the oxygen flow analyzer andthe second processor. In another aspect, the at least one processorcomprises a pressure sensor processor, and a second processor, thepressure sensor processor in communication with the pressure sensor andthe second processor. And in yet another aspect, the at least oneprocessor comprises an oxygen flow analyzer processor, a pressure sensorprocessor, and a third processor; the oxygen flow analyzer processor incommunication with the oxygen flow analyzer and the third processor, andthe pressure sensor processor in communication with the pressure sensorand the third processor.

According to another aspect, the processor may calculate a volume ofoxygen inhaled by a patient during at least one predetermined timeperiod. The steps may include the processor:

calculating a K-factor based on the oxygen flow rate value and theplurality of breathing pressure values, and calculating a portion of thebreathing pressure value due to a patient's breathing flow to determinea patient breathing flow value,

analyzing breathing pressure values to classify each as inhalation orexpiration, and for each breathing pressure value classified asinhalation, comparing the patient breathing flow value to the oxygenflow rate value,

-   -   and when the patient breathing flow value is greater than the        oxygen flow rate value, calculating the volume of oxygen due to        the oxygen flow rate during the at least one predetermined time        period as the volume of inhaled oxygen,    -   and when the patient's breathing flow value is less than the        oxygen flow rate value, calculating the volume of oxygen due to        the patient breathing flow rate during the at least one        predetermined time period as the volume of inhaled oxygen.

According to another aspect, an apparatus is described for identifyingthe source of oxygen delivery failure to a patient. The apparatus maycomprise:

a pressure sensor to detect a patient's breathing pressure and provide aplurality of breathing pressure values, and to measure ambient pressureand provide an ambient pressure value, the pressure sensor incommunication with a processor;

an oxygen flow analyzer to measure a parameter indicative of oxygen flowrate through an oxygen delivery device and provide an oxygen flow ratevalue, the oxygen flow analyzer in communication with the processor;

the processor programmed to receive the plurality of breathing pressurevalues, the ambient pressure value, and the oxygen flow rate value;

the processor programmed to analyze the plurality of breathing pressurevalues to determine if a patient is breathing or not breathing, and theprocessor further programmed to compare the plurality of breathingpressure values to the ambient pressure value when the processordetermines the patient is not breathing and the oxygen flow rate valueis greater than a predetermined threshold value; and

-   -   the processor programmed to output an apnea alarm if the        plurality of breathing pressure values is greater than the        ambient pressure value, and the processor programmed to output        an oxygen delivery device displacement alarm when the breathing        pressure differential value is equal to the ambient pressure        value.

In one configuration, the oxygen flow analyzer comprises adifferential-type oxygen flow analyzer. In another configuration, theoxygen flow analyzer comprises a heated wire-type anemometer.

In some configurations, the pressure sensor may be communication with asolenoid valve to switch the pressure sensor between measuring thepatient breathing pressure and measuring ambient pressure. Thepredetermined threshold value of the oxygen flow rate value may be, forexample, 0.5 liters per minute.

According to one aspect, the apnea alarm and the oxygen delivery devicealarm comprise an audible alarm. According to another aspect, the apneaalarm and the oxygen delivery device displacement alarm comprise avisual alarm.

According to another aspect, an apparatus to deliver a variable flow ofoxygen to a patient is provided. The apparatus may comprise:

a flow control valve to deliver a flow of oxygen through an oxygendelivery device;

a pressure sensor to detect the patient's breathing pressure and providea plurality of breathing pressure values, and to measure ambientpressure and provide an ambient pressure value, the pressure sensor incommunication with a processor;

the processor programmed to receive the plurality of breathing pressurevalues and the ambient pressure value;

the processor programmed to analyze the plurality of breathing pressurevalues to determine if a patient is breathing or not breathing, and theprocessor further programmed to calculate an amount of oxygen to deliverto a patient during a predetermined time frame, the amount of oxygen todeliver to the patient being calculated by a pre-selected oxygen flowrate plus an oxygen backlog; and

the processor programmed to open the flow control valve when theprocessor determines the patient is breathing in (inhaling) and theoxygen backlog is greater than zero.

The apparatus may further include an oxygen flow analyzer to measure aparameter indicative of oxygen flow rate through the oxygen deliverydevice and provide an oxygen flow rate value, the processor programmedto received the oxygen flow rate value. According to one aspect, theprocessor may be programmed to increment the oxygen backlog during thepredetermined time frame by the pre-selected oxygen flow rate multipliedby the predetermined time frame. The processor may be further programmedto decrement the oxygen backlog during the predetermined time frame whenthe processor determines the patient is breathing, such as the processordecrementing the oxygen backlog by the oxygen flow rate value multipliedby the breathing pressure value.

In one configuration, the flow control valve may include a proportionalvalve, and the processor may be programmed to send a current input tothe solenoid when the processor determines the patient is breathing. Theprocessor may calculate the current input to send to the solenoid bycomparing the breathing pressure value to the oxygen backlog, andsending the current input based on the smaller of the breathing pressurevalue and the oxygen backlog.

According to yet another aspect, there is provided a method fordelivering a variable flow of oxygen to a patient. The method maycomprise:

detecting a patient's breathing pressure value to determine a breathingpressure value;

receiving the breathing pressure value at at least one processor;

the at least one processor incrementing the total amount of oxygen to bedelivered based on a pre-selected oxygen flow rate and a predeterminedtime interval;

the at least one processor analyzing the breathing pressure value todetermine if the patient is inhaling or exhaling, and when the processordetermines the patient is inhaling, the at least one processorcalculating an optimal oxygen flow rate based on the total amount ofoxygen to be delivered and the breathing pressure value;

the at least one processor sending a signal to a flow control valve toopen the valve sufficiently to deliver the optimal oxygen flow rate;

measuring an oxygen flow rate through an oxygen delivery device toprovide an oxygen flow rate value;

receiving the oxygen flow rate value at the least one processor; and

the at least one processor decrementing the total amount of oxygen to bedelivered based on the oxygen flow rate value.

According to one aspect, the step of the at least one processorincrementing the total amount of oxygen to be delivered may comprise theat least one processor multiplying the pre-selected oxygen flow rate bythe predetermined time interval. The step of the at least one processoranalyzing the breathing pressure value to determine if the patient isinhaling or exhaling may comprise the at least one processor analyzingthe breathing pressure value to determine if it is negative or notnegative, and the at least one processor assuming a negative breathingpressure value is due to an inhalation. The step of the at least oneprocessor decrementing the total amount of oxygen to be delivered basedon the oxygen flow rate value may include the at least one processormultiplying the oxygen flow rate value by the predetermined timeinterval to calculate a calculated volume of oxygen delivered, and theat least one processor decrementing the total amount of oxygen to bedelivered by the calculated volume of oxygen delivered.

According to another aspect, there is provided a method for identifyinga source of oxygen delivery failure to a patient. The method maycomprise the steps of:

detecting a patient's breathing pressure value to determine a pluralityof breathing pressure values, and receiving the plurality of breathingpressure values at at least one processor;

the at least one processor analyzing the plurality of breathing pressurevalues to determine if the patient is breathing or not breathing, andwhen the processor determines the patient is not breathing, theprocessor communicating with a flow valve to deliver a high-flow pulseof oxygen;

measuring an oxygen flow rate through an oxygen delivery device toprovide an oxygen flow rate value;

receiving the oxygen flow rate value at the least one processor;

the processor analyzing the oxygen flow rate value, and wherein theprocessor outputs an insufficient oxygen flow alarm when the oxygen flowrate is not greater than a predetermined threshold value;

measuring a post-oxygen pulse breathing pressure value when the oxygenflow rate is less than the predetermined threshold value;

receiving the post-oxygen pulse breathing pressure value at the at leastone processor;

the processor analyzing the post-oxygen pulse breathing pressure value,and wherein the processor outputs an apnea alarm when the post-oxygenpulse breathing pressure value is more negative than a predeterminedthreshold post-oxygen pulse breathing pressure value, and wherein theprocessor outputs an oxygen delivery device displacement alarm when thepost-oxygen pulse breathing pressure values is equal to or less negativethan predetermined threshold post-oxygen pulse breathing pressure value.

According to one aspect, the step of measuring the oxygen flow ratethrough the oxygen delivery device may include measuring the oxygen flowrate with a differential-type oxygen flow analyzer. In someconfigurations, the step of measuring the oxygen flow rate through theoxygen delivery device may include measuring the oxygen flow rate with aheated wire-type anemometer. The steps of the processor outputting theapnea alarm and the oxygen delivery device displacement alarm mayinclude outputting an audible alarm and/or a visual alarm. In oneconfiguration, the high-flow pulse of oxygen is between 5 and 15 litersper minute. The predetermined threshold value may between 3 and 7 litersper minute in one configuration, and the predetermined thresholdpost-oxygen pulse breathing pressure value may be −0.1 cm H₂O.

According to one configuration, the at least one processor could be anoxygen flow analyzer processor and a second processor, the oxygen flowanalyzer processor in communication with the oxygen flow analyzer andthe second processor. In another configuration, the at least oneprocessor could be a pressure sensor processor and a second processor,the pressure sensor processor in communication with the pressure sensorand the second processor. In yet another configuration, the at least oneprocessor could be an oxygen flow analyzer processor, a pressure sensorprocessor, and a third processor; the oxygen flow analyzer processor incommunication with the oxygen flow analyzer and the third processor, andthe pressure sensor processor in communication with the pressure sensorand the third processor.

According to another aspect, the method may further comprise theprocessor calculating a volume of oxygen inhaled by the patient duringat least one predetermined time period, the processor analyzing theplurality of breathing pressure values to classify each as inhalation orexpiration, and for a breathing pressure value classified as inhalation,calculating the volume of inhaled oxygen as the volume of oxygen due tothe oxygen flow rate during the at least one predetermined time period.

According to another aspect, there is provided an apparatus foridentifying a source of oxygen delivery failure to a patient. Theapparatus may include:

a pressure sensor to detect a patient's breathing pressure and provide aplurality of breathing pressure values, and provide a post-oxygen pulsebreathing pressure value, the pressure sensor in communication with aprocessor;

a variable flow valve to deliver a high-flow pulse of oxygen, thevariable flow valve in communication with the processor;

an oxygen flow analyzer to measure a parameter indicative of oxygen flowrate through an oxygen delivery device and provide an oxygen flow ratevalue, the oxygen flow analyzer in communication with the processor;

the processor programmed to receive the plurality of breathing pressurevalues and to analyze the plurality of breathing pressure values todetermine if the patient is breathing or not breathing, and theprocessor programmed to send a signal to the variable flow valve to openthe valve to deliver the high-flow pulse of oxygen when the processordetermines the patient is not breathing; and

the processor programmed to receive the oxygen flow rate value andanalyze the oxygen flow rate value to determine if the oxygen flow ratevalue is above a predetermined threshold value, and the processorfurther programmed to output an insufficient oxygen flow alarm when theoxygen flow rate value is below the predetermined threshold value;

the processor programmed to receive the post-oxygen pulse breathingpressure value and analyze the post-oxygen pulse breathing pressurevalue to determine if the post-oxygen pulse breathing pressure value ismore negative than a predetermined threshold post-oxygen pulse breathingpressure value; and

the processor programmed to output an apnea alarm if the post-oxygenpulse breathing pressure value is more negative than the predeterminedthreshold post-oxygen pulse breathing pressure value, and the processorprogrammed to output an oxygen delivery device displacement alarm whenthe post-oxygen pulse breathing pressure value is not more negative thanthe predetermined threshold post-oxygen pulse breathing pressure value.

In one configuration, the oxygen flow analyzer comprises adifferential-type oxygen flow analyzer. In another configuration, theoxygen flow analyzer comprises a heated wire, or heated film,-typeanemometer. The variable flow valve could be a proportional controlvalve. Depending on the configuration, different values may be used forthe high-flow pulse of oxygen, and in one configuration, the pulse maybe 5 to 15 liters per minute. The predetermined threshold value of theoxygen flow rate value may be 4 to 10 liters per minute.

In some configurations, the apnea alarm and the oxygen delivery devicedisplacement alarm may include an audible and/or visual alarm.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings illustrate what are currently considered to bespecific configurations for carrying out the invention.

FIG. 1 is a diagram of a system in clinical practice that can determinethe source of oxygen delivery failure to a patient and/or calculate thefraction of delivered oxygen that is inhaled by a patient.

FIG. 2 is a flow chart illustrating the logic to determine whether apatient is breathing or not.

FIG. 3 is a flow chart illustrating the logic to determine the source ofoxygen delivery failure to a patient.

FIG. 4 is a flow chart illustrating another logic that may be used todetermine the source of oxygen delivery failure to a patient.

FIG. 5 is a diagram of an apparatus to determine the source of oxygendelivery failure to a patient and/or calculate the fraction of deliveredoxygen that is inhaled by a patient.

FIG. 6 is a flow chart illustrating the logic to calculate a volume ofdelivered oxygen that is inhaled by a patient.

FIG. 7 is a flow chart illustrating another logic that may be used tocalculate a volume of delivered oxygen that is inhaled by a patient.

FIG. 8 is a flow chart illustrating the logic to calculate the totalvolume of oxygen to be delivered to a patient.

FIG. 9 is a flow chart illustrating the logic to deliver a variable flowof oxygen to a patient based on the total volume of oxygen to bedelivered to the patient.

DETAILED DESCRIPTION

Reference will now be made to the drawings in which the various elementsof the illustrated configurations will be given numerical designationsand in which the invention will be discussed so as to enable one skilledin the art to make and use the invention. It is to be understood thatthe following description is only exemplary of the principles of thepresent invention, and should not be viewed as narrowing the claimswhich follow.

Definitions

The term “oxygen delivery failure” means the patient is failing toreceive oxygen. The threshold for oxygen delivery failure could be setby a clinician, for example, failure to receive a minimum fraction ofthe set oxygen delivery rate, such as 50% of the oxygen delivery rate.This cause of the oxygen delivery failure could be a failure of theoxygen delivery device, such as if the oxygen delivery device isdisplaced or if the oxygen supply is insufficient. Oxygen deliveryfailure could also be caused by a patient's apnea, or failure to breath.

The term “breathing pressure” means the pressure that is sensed by thepressure sensor. This pressure is typically primarily caused by apatient's breathing, with inhalations detected as a negative pressureand exhalations detected as a positive pressure. Breathing pressure canalso be caused by the flow of oxygen in an oxygen delivery device,and/or by the ambient pressure.

The term “logic” means an algorithm or a step-by-step method of solvinga problem or making decisions. For clarity, the logic herein has beengenerally shown and described in a step-by-step manner with a particularorder. However, the particular order shown is illustrative and notlimiting. Some of the steps of the logic shown may be performed in theparticular order shown, or some of the steps may be performed in adifferent order, or some of the steps may be performed at the same time.

The term “processor” means a standard processor, typically equipped witha control unit, a logic unit, and a register.

A perspective view of the apparatus for identifying the source of oxygendelivery failure to a patient, as it may be used in clinical practice,is shown in FIG. 1. In clinical practice, the patient is provided withan oxygen delivery device 17. The oxygen delivery device 17 may be anasal cannula, as shown in FIG. 1, such as a single- or double-narednasal cannula, or it may be a mask or other device. The oxygen deliverydevice 17 is provided with a connection, such as tubing, to an oxygensource 22. It may be desirable to provide a flow control valve 25 inconnection with the oxygen source 22 so the medical personnel canselect, adjust, or control the oxygen flow rate from the oxygen source22.

The flow control valve 25 may be a simple needle valve. The flow controlvalve may be a simple ON/OFF type valve that has only two positions:open and closed. In one configuration, the flow control valve 25 mayprovide a variable flow. The flow control valve 25 may be connected tothe processor 35 and may deliver a variable gas flow. For example theprocessor may set a flow rate that changes up to 100 times per secondaccording to variables such as the desired oxygen delivery for theparticular patient and the measure oxygen delivery to the patient. Thevariable-type of flow control valve 25 may be, for example, those knownin the art such as the EVP Series Proportional Control Valvesmanufactured by Clippard Instrument Laboratory, Inc.

Additionally, at some point in the tubing between the patient 14 and theoxygen source 22, an oxygen flow analyzer 30 may be provided. The oxygenflow analyzer 30 measures an oxygen flow rate to provide an oxygen flowrate value. The oxygen flow analyzer 30 may be of any type known in theart, and in one configuration may be a simple differential pressuresystem. For example, a slight obstruction, such as a section of tubinghaving a smaller inner diameter 32, is placed in the oxygen flow tubingwithin the system, and a differential pressure analyzer measures thepressure difference caused by the flow interruption. The measureddifferential pressure is then used to calculate the corresponding oxygenflow rate. Another type of suitable oxygen flow analyzer 30 could be aheated wire-type anemometer. It may be desirable to include a barometerin the system and compensate for the change in gas density and variousaltitudes.

The oxygen flow analyzer 30 may be equipped with its own oxygen flowanalyzer processor 34 to calculate the oxygen flow rate value from themeasured parameters. The oxygen flow analyzer processor 34 may thencommunicate the oxygen flow rate value to a processor 35. Alternatively,the oxygen flow analyzer 30 may be in communication with a processor 35to provide the processor 35 with the measured parameters necessary forthe processor 35 to calculate the oxygen flow rate value. The processor35 may be programmed to store and analyze the oxygen flow rate valuesreceived from the oxygen flow analyzer processor 34 or the oxygen flowrate values calculated by the processor 35.

The apparatus for identifying the source of oxygen delivery failure to apatient may also include a pressure sensor 40. The pressure sensor 40may be a precision, low-pressure sensor. The pressure sensor 40 is incommunication with the patient such that the pressure sensor 40 detectsa patient's breathing pressure. For example, if the patient is using amask as an oxygen delivery device 17, the pressure sensor 40 may be incommunication with the mask by tubing 42. In the case of a nasal cannulaas the oxygen delivery device 17, a piece of tubing 42 in communicationwith the pressure sensor may be placed in one (or both) of the patient'snostrils. In one configuration, the pressure sensor is provided with aport that allows simple insertion of a piece of tubing 42. The tubing 42can then be placed either on the patient's mask or in a patient'snostril(s). Additionally, the pressure sensor 40 may be disposedanywhere along the tubing it is desired. For example, the pressuresensor 40 may be placed very close to a patient's oxygen delivery device17, or the pressure sensor 40 may be placed farther from the patient andcloser to the medical personnel and other medical instruments.

The pressure sensor 40 detects a patient's breathing pressure, includingthe inhalations, which are detected as a negative pressure, and theexhalations, which are detected as a positive pressure. The plurality ofbreathing pressure values measured by the pressure sensor 40 may beanalyzed by a pressure sensor processor 45 and communicated to theprocessor 35. Alternatively, the plurality of breathing pressure valuesmeasured by the pressure sensor 40 may be communicated to the processor35 and the processor 35 may analyze the breathing pressure values.

The apparatus for identifying the source of oxygen delivery failure to apatient may also be provided with a device to measure the ambientpressure. In one configuration, this may be a separate pressure sensorto measure ambient pressure. In another configuration, the pressuresensor 40 is in communication with a solenoid valve 48. Either theprocessor 35 or a pressure sensor processor 45 may direct the openingand closing of the solenoid valve 48. Typically, the solenoid valve 48will remain closed such that the pressure sensor 40 is measuring thepatient's breathing pressure. Periodically, the solenoid valve 48 may beopened, and the pressure sensor 40 would then measure the ambientpressure. (The solenoid valve 48 could also be configured such that theopen position measures the patient breathing pressure and the closedposition measures the ambient pressure.) The ambient pressuremeasurement may be used to determine an ambient pressure value. Again,this may be achieved through the use of a separate processor, such asthe pressure sensor processor 45, with the pressure sensor processor 45communicating the ambient pressure value to the processor 35;alternatively, the ambient pressure measurement may be communicated tothe processor 35, and the processor 35 may determine the ambientpressure value.

The processor 35 may receive data from the oxygen flow analyzer 30 (orfrom the oxygen flow analyzer processor 34) and the pressure sensor 40(or the pressure sensor processor 45). The processor may be providedwith one or more input ports to receive data, and one or more outputports. The processor may be programmed to output an apnea alarm, aninsufficient oxygen flow alarm, or an oxygen delivery devicedisplacement alarm. These alarms may be, for example, audible alarms,visual alarms, or both. The processor 35 may output, for example, to aspeaker 53 and/or a visual indicator. In one configuration, theprocessor is in communication with a display screen 58 that allows auser to select parameters for the processor. The processor may alsooutput alarms and data to the display screen.

The processor 35 analyzes that data it receives to determine the sourceof oxygen failure to a patient. The logic of the processor comprises twogeneral steps: (1) analyzing the plurality of breathing pressure valuesto determine if a patient is breathing, and (2) comparing the pluralityof breathing pressure values to the ambient pressure value when theprocessor determines the patient is not breathing and the oxygen flowrate value is greater than a predetermined threshold.

In the first step, the logic of the processor 35 analyzing the pluralityof breathing pressure values to determine if a patient is breathing, thelogic comprise any suitable logic, such as those already known in theart. FIG. 2 is a flow chart that describes an operable method to analyzethe plurality of breathing pressure values to determine if a patient isbreathing. The processor may first calculate a baseline pressure 50. Thebaseline pressure may be calculated by measuring the pressure at thetime between a patient's breaths, i.e., the pause between inspirationand expiration. At that time, the only measured pressure should ideallybe a slight positive pressure due to the flow of oxygen. Other suitablemethods to calculate the baseline pressure include calculating theaverage of the pressure values during a predetermined time period,calculating the median pressure value during a predetermined timeperiod, etc.

Once the processor determines a baseline pressure, the processor maycalculate an upper threshold value and lower threshold value by addingand subtracting, respectively, a small offset, such as 0.01 cm H₂O,shown in FIG. 2 at 52. The processor then observes each breathingpressure value and categorizes it as an expiration, inspiration, orinsignificant (54). For example the processor may receive one breathingpressure value every 10 milliseconds (100 breathing pressure values persecond). Each breathing pressure value may be analyzed and categorized.To determine if a patient is breathing, the processor checks for both aninspiration and an expiration during a pre-determined time period (56).This pre-determined time period may be, for example, 10 seconds. In oneconfiguration, the processor may have an interface, such as a displayscreen, to allow medical personnel to select the pre-determined timeperiod they desire for a particular patient. As shown in FIG. 2, if bothan inspiration and an expiration are detected during the pre-determinedtime period, the patient is breathing, and the processor continues toanalyze breathing pressure values over the next pre-determined timeperiod. If both an inspiration and an expiration are not detected duringthe pre-determined time range, then the patient is not breathing, andthe processor continues to the logic to identify the source of oxygendelivery failure to the patient (58).

The logic for identifying the source of oxygen delivery failure to apatient may include the steps shown in FIG. 3. While the logic shown inFIGS. 2-4 shows discrete steps taken in order, the steps need not betaken in the particular order shown. For example, the processor may beprogrammed to continuously check the oxygen flow rate value and triggeran oxygen flow rate alarm at any time the oxygen flow rate value is lessthan a predetermined threshold. Similarly, the processor may beprogrammed to periodically (or continuously) sample the ambientpressure. The steps may be taken in a different order and still achievethe desired outcome. In FIG. 3, after the processor logic has determinedthe patient is not breathing (60), the processor may determine if thereis sufficient oxygen flow. This may be done, for example, by theprocessor analyzing the oxygen flow rate value to determine if it isgreater than a predetermined threshold value (62). In one configuration,this predetermined threshold value may be 0.5 liters per minute. Inanother configuration, the processor 35 may include an interface whichallows medical personnel to select the desired predetermined thresholdvalue for oxygen flow rate. If the processor determines that the oxygenflow rate value is less the predetermined threshold value, the processormay trigger an insufficient oxygen flow alarm (64). This alarm may be anaudible alarm, a visual alarm, or it may be both audible and visual. Ifthe processor determines that the oxygen flow rate value is greater thanthe predetermined threshold value, the processor further compares theplurality of breathing pressure values to the ambient pressure value(66). The processor may compare the plurality of breathing pressurevalues to a stored ambient pressure value, or the processor may beprogrammed to sample the ambient pressure value when it is determinedthat a patient is not breathing and the oxygen flow rate value isgreater than the predetermined threshold value. When the processordetermines the plurality of breathing pressure values are greater thanthe ambient pressure value, the processor outputs an apnea alarm (68).When the processor determines the plurality of breathing pressure valuesare equal to the ambient pressure value, the processor outputs an oxygendelivery device displacement alarm (70).

The apnea alarm and the oxygen delivery device displacement alarm may beaudible alarms, visual alarms, or both. In one configuration, the apneaalarm may be louder and/or have greater visibility than the oxygendelivery device displacement alarm. This configuration may be preferredsince apnea is an immediate life-threatening danger and requires medicalpersonnel to respond immediately, whereas oxygen delivery devicedisplacement may be less hazardous.

In another configuration, the processor may determine whether there isinsufficient oxygen flow by measuring the oxygen flow rate in responseto a known voltage on the flow control valve 25 to determine if there isan insufficient oxygen flow. This configuration is shown in FIG. 4. Theprocessor may first determine that there is an oxygen delivery failure(60). This may be done by performing the steps indicated in FIG. 2, orthe processor may be programmed to routinely check for oxygen deliveryfailure. When the processor determines there is an oxygen deliveryfailure (60), the processor 35 sends a signal to the flow control valve25 to increase the voltage on the flow control valve to a known, highvoltage to deliver a high-flow pulse of oxygen (for example, 5-15Liters/min for 50-200 milliseconds)(150). The system then measures theoxygen flow rate value in response to the high-flow pulse of oxygen(152). When oxygen flow is sufficient and there are no issues with theoxygen delivery, the increased voltage on the flow control valve 25should result in an increase in the measured oxygen flow by the oxygenflow analyzer 30. The system may be programmed to have a predeterminedthreshold valve for the oxygen flow rate value in response to thevoltage (for example, an oxygen flow rate value of 5 L/min). If theoxygen flow analyzer 30 measures an oxygen flow rate value above thepredetermined threshold value, due to the increase in voltage on theflow control valve, there is not an issue with oxygen delivery. If theprocessor sends a signal to the flow control valve 25 to increase thevoltage and the oxygen flow analyzer 30 does not measure an oxygen flowrate value above the predetermined threshold value, there is a problemwith oxygen delivery and the system may trigger an insufficient oxygenflow alarm (64).

If there is sufficient oxygen flow in response to the high-flow pulse ofoxygen, the processor may then analyze the breathing pressure value,post-oxygen pulse (taken by pressure sensor 40) to determine if thepost-oxygen pulse breathing pressure value is greater than apredetermined threshold value (154). If the oxygen delivery device 17 isin place, the high-flow pulse of oxygen should cause an increasedpositive pressure at the oxygen delivery device 17. A pre-determinedthreshold value for the post-oxygen pulse breathing pressure value inresponse to the high-flow pulse of oxygen may be programmed into theprocessor 35, and the post-oxygen pulse breathing pressure value may becompared to the pre-determined threshold value. If the post-oxygen pulsebreathing pressure value is less negative, it is an indication that theoxygen is being lost into the ambient pressure of the room, and anoxygen delivery device displacement alarm may be triggered (70). If thepost-oxygen pulse breathing pressure value is more negative, it is anindication that the patient is receiving the high-flow pulse of oxygenand not breathing, and an apnea alarm (68) may be triggered.

In one configuration, the processor, oxygen flow analyzer, and pressuresensor are contained in a single housing, such as that shown in FIG. 5.The housing 80 comprises an input port 81 for receiving tubing (notshown) from an oxygen source, and an output port 85 for the oxygen thatflows into the housing and through the oxygen flow analyzer 30 to flowto the oxygen delivery device of the patient. The oxygen may flow intothe housing from an oxygen source in the direction of the arrow 83 andout of the housing to the patient delivery device in the direction ofthe arrow 87. The housing also comprises a port 92 for a tube to beconnected from the oxygen delivery device of the patient to the pressuresensor 40 located in the housing. This may also be in communication witha solenoid valve 48, such that the pressure sensor 40 may detect thepressure of the patient breathing or the ambient pressure.

The housing may include an interface, such as a screen 58, for medicalpersonnel to set specific parameters of operation, such as what timeincrements the breathing pressure sensor will sample for breathingpressure values, what time increments the solenoid valve will open orclose to sample the ambient pressure, the predetermined threshold valuefor oxygen flow, and/or the predetermined time period for one inhalationand one exhalation to be detected in order to determine the patient isbreathing. In some configurations, the medical personnel can select tohave these parameters set as defaults that will work for a majority ofpatients. In another configuration, one or more of these parameters maybe specifically selected by medical personnel for the needs of aparticular patient.

According to another aspect, the method may include the processorcalculating a volume of oxygen inhaled by a patient during apredetermined time period. The processor 35 may be programmed tocalculate the volume of inhaled oxygen using the formula

flow=K√{square root over (pressure)}

In this formula, there is a square law relationship between flow andpressure. The factor K accounts for the gas flow resistance created bythe particular patient's nostril and the placement of the cannula withinthe nostril (or, in the case of a mask, the placement of the mask on thepatient). It is known that for any orifice defining a pressure dropbetween a pressurized system and a lower pressure surroundingenvironment (i.e., ambient pressure), the flow through the orifice willbe proportional to the square root of the pressure difference across theorifice multiplied by a constant K which characterizes the mechanicalfeatures of the orifice itself, that is its size, surface smoothness,and so forth. K is not a constant and is normally not known because itvaries from patient to patient, but may be calculated with the currentsystem. Flow is the calculated flow rate of the oxygen flowing past theoxygen delivery device (either a mask or a cannula). Pressure is thepressure difference between the ambient pressure measured by openingsolenoid valve 48 on pressure sensor 40, and the breathing pressure atthe oxygen delivery device 17 measured by pressure sensor 40 (breathingpressure at the oxygen delivery device 17 is caused by both thepatient's breathing pressure and the oxygen flow pressure).

Within the system, the oxygen flow is being determined by the oxygenflow analyzer 30, and the pressure sensor 40 allows the determination ofthe baseline pressure caused by the oxygen flow (as described above).When a patient is not breathing and there is no oxygen flow, the oxygenflow and the baseline pressure should both be zero. Typically, a patienthas a pause at the end of expiration before the next inhalation begins.At this pause, the only source of pressure detected by the pressuresensor 40 is from the oxygen flow (rather than pressure detected fromoxygen flow plus a patient's breathing). And the oxygen flow that isinducing the non-zero pressure is known since the oxygen flow isdirectly measured in the system by the oxygen flow analyzer 30. Thus, Kmay be calculated at the pause, where flow and pressure are known, using

$K = \frac{flow}{\left. \sqrt{}{pressure} \right.}$

Flow here is the oxygen flow value determined by the oxygen flowanalyzer 30, and the pressure is the breathing pressure value determinedby the pressure sensor 40. The factor K can then be stored in theprocessor 35. The stored K-factor can be used in the equationflow=K√{square root over (pressure)} to determine the patient'sbreathing flow value from the pressure signal for every pressure signalsampled.

The system also may assume that no delivered oxygen is being inhaledwhile a patient is exhaling. Likewise, the system may assume that alldelivered oxygen is being inhaled when a patient is inhaling at apatient breathing flow greater than the delivered oxygen flow. As shownin the logic of FIG. 6, the processor may first calculate the K-factor,as described above (94). The processor may analyze breathing pressurevalues received to classify each as an inhalation or expiration (96).Based on the assumption that no delivered oxygen is being inhaled whilea patient is exhaling, expirations are insignificant in the calculationof inhaled oxygen (98). For each breathing pressure value classified asan inhalation, the system may use the equation above to calculate aportion of the breathing pressure value due to a patient's breathingflow to determine a patient breathing flow rate value (100).

The processor may then compare the patient breathing flow rate value tothe oxygen flow rate value (100). When the patient breathing flow ratevalue is greater than the oxygen flow rate value, the processor maycalculate the volume of inhaled oxygen as the volume of oxygen due tothe oxygen flow rate during the predetermined time period (102). Whenthe patient's breathing flow rate value is less than the oxygen flowrate value, the processor may calculate the volume of inhaled oxygen asthe volume of oxygen due to the patient breathing flow rate during thetime period (104). For each breathing pressure value classified as aninhalation, the processor may perform the steps above to determine thevolume of inhaled oxygen during the time period. The processor may beprogrammed to further add the volume of inhaled oxygen during multipletime periods together to determine a volume of inhaled oxygen over alonger time period. Moreover, the steps to calculate the volume ofinhaled oxygen need not be performed in the particular order shown inFIG. 6. For example, the processor 35 may first classify a breathingpressure value as an inhalation or exhalation, and then determine theK-factor. Likewise, these steps may be performed simultaneously by theprocessor 35.

Turning now to FIG. 7, there is shown another example of logic that maybe used to calculate the volume of inhaled oxygen. It should beappreciated that while the steps in FIG. 7 are shown in a particularorder, the steps may be performed in a different order. Similarly, whilethe steps are shown separately, one or more steps may be performed atthe same time. According to this logic, the processor 35 analyzes thebreathing pressure value from the pressure sensor 40 to determine if thebreathing pressure value is associated with an inhalation (110). Thismay be done, for example, by comparing the breathing pressure value witha pre-determined threshold value. If the breathing pressure value islower (or more negative), the patient is inhaling. The processor mayalso be programmed to associate any negative breathing pressure value asan inhalation.

If the breathing pressure value is not associated with an inhalation,the patient is determined to be exhaling, and exhalation isinsignificant to the calculation of the volume of inhaled oxygen (112).If the breathing pressure value is associated with an inhalation, theprocessor 35 may be programmed to assume that the volume of inhaledoxygen during the predetermined time period is due to the oxygen flowrate (i.e., the oxygen flow rate for the predetermined time period, asmeasured by the oxygen flow analyzer 30, multiplied by the predeterminedtime period)(114).

FIG. 8 shows the logic the processor 35 may follow to calculate a totalvolume of inhaled oxygen over a greater time period (for example, 1minute) by calculating the volume of oxygen inhaled during eachpredetermined time period (for example, 1 millisecond) within thegreater time period, and adding them together. The processor starts(116) and samples the oxygen flow rate value from the oxygen flow rateanalyzer 30 (118). The processor then uses this oxygen flow rate valueto calculate the volume of oxygen delivered for the predetermined timeperiod (120), by multiplying the oxygen flow rate value by thepredetermined time period (for example, an oxygen flow rate value of 6L/min times a predetermined time period of 10 milliseconds, for a volumeof 1 mL of oxygen delivered).

The processor then adds the volume of oxygen delivered for thepredetermined time period to the total volume of oxygen delivered (122).The total volume of oxygen delivered is incremented each predeterminedtime period by the volume of oxygen delivered during that predeterminedtime period. The processor then analyzes the breathing pressure valuefrom the pressure sensor 40 to determine if the breathing pressure valueis associated with an inhalation (124), which may be done by the methodsdescribed above (i.e., determining if the breathing pressure value isbelow a threshold value or if the breathing pressure value is negative,etc.).

If the breathing pressure value is not associated with an inhalation, nooxygen is delivered during exhalation (126), and the processor willdetermine if the period is complete (132). If the period is notcomplete, the algorithm continues by sampling the oxygen flow rate valuefor the next predetermined time period (118) and continuing through theloop. If the breathing pressure value is associated with an inhalation,the processor calculates the volume of inhaled oxygen during thepredetermined time period (128). This may be done, for example, usingthe steps outlined in FIG. 6 or FIG. 7 (using the predetermined flowrate, either from the oxygen flow rate or the patient breathing flowrate, multiplied by the predetermined time period). The processor thenadds the volume of inhaled oxygen for the predetermined time period tothe total volume of inhaled oxygen (130). The processor then determinesif the period is complete (132)(i.e., if the time period, such as oneminute, is over). If not, the algorithm continues by sampling the oxygenflow rate value for the next predetermined time period (118) andcontinues the loop until the period is over.

When the period is complete, the processor may be programmed to thencompare the total volume of inhaled oxygen to the total volume ofdelivered oxygen (134). The processor may output a percentage of inhaledoxygen. This output may be displayed to clinicians on a display screen(FIG. 5). The clinician may be able to select what parameters of data todisplay, for example, what fraction of oxygen is inhaled, what fractionof oxygen is wasted, etc.

According to another aspect, the system described herein may be used tocontrol oxygen delivery in a specific manner. By using the pressuresensor 40 to determine when, and at what rate, the patient is inhaling,the system may deliver a targeted amount of oxygen to the patient.Moreover, the flow control valve 25 may adjust the oxygen flow rapidlyin response to the patient's sensed breathing pressure, and provide avariable oxygen flow rather than merely an “on/off” control. This mayhave the advantage of reducing wasted delivered oxygen (when thedelivered oxygen is not inhaled but rather lost to the ambient air) andalso increasing the amount of oxygen actually delivered to the patient.This may also reduce the flammability risk that is inherent in the useof oxygen delivery.

The flow control valve 25 may be any suitable valve known in the art,such as the EPV proportional control valves manufactured by Clippardmentioned above. These valves have a solenoid, and are capable ofvarying oxygen flow based on the current input to the solenoid. Greatercurrent input to the solenoid opens the valve further, and a precisedegree of control over the flow rate is possible. A single flow controlvalve 25 may be used, or more than one flow control valve can be used inconjunction.

In one configuration, the processor may first optimize the oxygen flowfor the particular patient such that pressure at the oxygen deliverydevice equals the ambient pressure in the room. (Except possibly incases of deep inhalations, where it is clinically impractical to deliversuch a high oxygen flow, and usually not necessary for a patient who istaking deep inhalations.)

The ambient pressure in the room may be measured by the opening thesolenoid valve 48 on the pressure sensor 40. Alternatively, the ambientpressure may be measured by an ambient pressure sensor 44 incommunication with the processor 35. The ambient pressure value is sentto the processor 35. The processor uses the ambient pressure value todetermine an optimal oxygen flow value (which is equal to the ambientpressure value). The processor then sends a signal to the flow controlvalve 25 to increase or decrease the oxygen flow, and this process iscontinued until the oxygen flow is optimized for the particular patient,that is, until the pressure at the oxygen delivery device 17 equals theambient pressure. If the pressure at the oxygen delivery device 17 isany greater, i.e., the pressure is positive, and oxygen is lost into theroom. If the pressure is lower, a critical opportunity to deliver oxygento the patient is lost. It may not be possible to deliver a flow ofoxygen sufficiently high at the start of the breath if the patient istaking deep breaths, but the processor may send a signal at that pointto deliver the maximum flow rate possible.

The processor continues to send the signal to the flow control valve 25to maintain the flow of oxygen (such that the pressure at the oxygendelivery device 17 equals the ambient pressure) until the predeterminedamount of oxygen has been inhaled for that breath, or until the pressuresensor 40 indicates the patient has started to exhale. No oxygen isdelivered during exhalation. Patients in opioid induced respiratorydepression have short periods of inspiration relative to long pauseperiods between breaths. By delivering the highest flow of oxygen at thestart of inspiration, oxygen flow is optimized so that it is most likelyto be drawn deep into the lungs.

According to another aspect, the amount of oxygen delivered to thepatient may be based on a calculation of the oxygen they have alreadyinhaled and the amount of oxygen the patient still needs to inhale basedon parameters set by the clinician. In this configuration, the clinicianmay determine a set amount of oxygen flow that they desire the patientto receive, based on the particular patient's current condition. This isthe “pre-selected oxygen flow rate,” and may be optimized for eachpatient. For example, it may be desired that the patient receive a flowof oxygen of 6 liters per minute (6 L/min). However, merely setting atraditional oxygen delivery device to 6 L/min does not ensure that thepatient receives all the oxygen that should be delivered at this rate.According to the principles taught herein, the flow control valve candeliver precise amounts of oxygen in a given time interval to ensure thepatient receives the desired amount of oxygen. The pre-determined timeinterval may be set by the clinician via the processor 35. By way ofexample, the pre-determined time interval may be 10 milliseconds.

The processor 35 may perform an algorithm for every pre-determined timeinterval (for example, every 10 milliseconds) to determine the preciseflow to deliver for that time interval. The algorithm may include thesteps shown in FIG. 9. It should be appreciated that while the steps inFIG. 9 are shown in a particular order, the steps may be performed in adifferent order. Similarly, while the steps are shown separately, one ormore steps may be performed at the same time.

In one configuration, the processor 35 may automatically adjust toaccount for that fact that a typical patient is only inhaling during onethird of the time. So if the clinician selected a flow rate of 6 L/min,the processor 35 may assume that the patient only inhales one-third ofthe time, and will set a rate of oxygen delivery of 2 L/min. In otherconfigurations, the processor may be set to calculate the oxygenincrement with or without this assumption.

The amount of oxygen the processor 35 calculates to be delivered in anygiven time interval is the amount of oxygen in the oxygen backlog. Theoxygen backlog is the amount of oxygen that is to be delivered, but hasnot yet been delivered. For each predetermined time interval, the oxygenbacklog is incremented or increased by at least the oxygen to bedelivered based on the clinician's pre-selected oxygen flow rate (136 inFIG. 9). Oxygen to be delivered based on the pre-selected flow rate iscalculated as follows: the set flow rate (i.e. 2 L/min) multiplied bythe predetermined time interval (i.e. 10 mSec, or 6000 per minute). Forexample, where the pre-selected oxygen flow rate is 2 L/min, or 2000mL/minute, and the predetermined time interval is 10 mSec, it would be2000 mL/min multiplied by 10 mSec (or 2000 mL/min divided by 6000predetermined time intervals per minute), resulting in a delivery of0.333 mL per predetermined time interval. This is the amount of oxygenthe processor 35 would calculate to deliver per time interval based onthe pre-selected flow rate. Each time the processor 35 performs thealgorithm shown in FIG. 9, or in other words, for each loop through thealgorithm, the processor increments the oxygen backlog, or the oxygen tobe delivered by this amount. Thus, for every predetermined timeinterval, the oxygen to be delivered is increased or incremented by thisamount.

The processor 35 may also be programmed to track any backlog of oxygenthat is not delivered during a predetermined time interval, and furtherincrement the oxygen backlog for the next predetermined time interval bythis amount. The processor may analyze the patient's breathing pressurevalue to determine if the breathing pressure value is negative orpositive (138). (The processor may also determine if the patient isinhaling or exhaling by assuming that a breathing pressure value withinthe oxygen delivery device 17 above a pre-determined threshold value isan exhalation.) If the patient's breathing pressure is positive(exhale), the system is programmed to not deliver oxygen during anexhale (140), and the oxygen backlog for that predetermined timeinterval will not be delivered during that predetermined time interval.In the above example, for every predetermined time interval where thepatient is exhaling, the oxygen backlog would be incremented by 0.333mL. Or for each loop (from 136 to 138 to 140, and back to 136) throughthe algorithm during which the breathing pressure is positive, theoxygen backlog would be incremented by 0.333 mL.

In another example, if the clinician selected a flow rate of 2 L/min,the processor may automatically assume that only 666 mL would actuallybe inhaled, and adjust the set flow rate down accordingly. In thisexample with a flow rate of 2 L/min, the oxygen increment for eachpredetermined time interval (where a predetermined time interval is 10mSec) would be 0.111 mL.

When the processor determines the patient is breathing, the processormay calculate an optimal oxygen flow rate based on the oxygen to bedelivered (i.e., the oxygen backlog) and the measured breathing pressurevalue (142). The optimal oxygen flow rate is based on the followingcalculation:

Optimal O2 flow rate=−breathing pressure value×factor×oxygen backlog

Where breathing pressure value is the breathing pressure value measuredby the pressure sensor 40, the factor is a tuning constant based on thepatient size, etc., and the oxygen backlog is the total amount of oxygenthat needs to be delivered to the patient. As can be seen by thisequation, the flow rate is based on the breathing pressure value, andwill increase as the patient breathes deeper, and decrease as thepatient's breath becomes more shallow (as at the end of a breath). Thus,in one breath, the optimal oxygen flow slows as the patient's ownbreathing slows. Similarly, the equation shows that the optimal oxygenflow is based on the oxygen backlog. As the backlog increases, theoptimal oxygen flow rate will also increase.

The processor then opens the flow control valve to deliver thecalculated optimal oxygen flow rate (144). This can be done bydelivering a signal to the flow control valve using software (thesoftware may be either integral to the valve, or integral to theprocessor 35) with a digital to analog converter that creates a voltagethat is proportional to the digital value generated by optimal flow ratecalculation. The voltage from the converter is fed into an analogcircuit which generates a proportional electrical current that drivesthe valve. The volume of oxygen that flows through the valve during thesample period is directly proportional to the digital to analogconvertor unit value. In one configuration, each converter unitcorresponds to 0.005 mL of volume per sample period.

The breathing pressure value may be measured, for example, using adigital pressure sensor (for example, the DLVR models manufactured byAll Sensors, Inc.), that converts the actual pressure in cm H2O intodigital to analog converter units where 1 convertor unit is equal to0.00031 cm H2O. To reduce the number of calculations, the algorithm maycombine the unit conversion factors into the constant proportionalcontrol factor. This gives a control equation that is implemented insoftware as follows:

Digital to analog convertor units output=AD convertor units×oxygenbacklog×K

Where digital to analog convertor units output is the digital to analogconverter value that is used to generate the valve current and controlthe delivered oxygen rate; AD converter units is the digital output ofthe digital pressure sensor (where one unit equals 0.00031 cm H2O in theexemplary configuration above); Oxygen backlog is the total amount ofoxygen to be delivered, and K is the constant proportional controlfactor combined with needed conversion factors (where the factor K is0.05 in the exemplary configuration above).

During an individual inhalation, the algorithm is repeated for everypredetermined time interval, and as the oxygen is actually delivered tothe patient, the oxygen backlog is decreased by the amount of oxygendelivered to the patient for each predetermined time interval (146 inFIG. 9). Thus, the oxygen to be delivered, or the oxygen backlog, isdecreased over the course of an inhalation. As the oxygen backlog isdecreased, the flow of oxygen decreases, and over the course of theinhalation, the oxygen flow decreases and it is less likely thatdelivered oxygen will be left in the trachea at the end of inhalation.

Additionally, the processor may be programmed to integrate the totalvolume of oxygen inhaled over the course of a time period to alert theclinician if the total volume of oxygen inhaled is sufficient or notsufficient. For example, the processor may integrate the volume ofoxygen that is actually delivered to the patient over the course of oneminute. If there is insufficient oxygen delivered over the course of theone minute, the processor may notify the clinician of insufficientoxygen delivery through an alarm, such as an audible alarm and/or anon-screen message.

The oxygen delivery algorithm shown in FIG. 9 achieves the desired goalsof increasing the amount of oxygen actually delivered to the patient,and decreasing the amount of wasted oxygen. For example, if thepatient's breathing rate slows, there will be fewer predetermined timeintervals wherein the breathing pressure value is less than zero (fewerinhalations). As the breathing rate slows, the oxygen backlog increases,because for every predetermined time interval, the algorithm isincrementing the amount of oxygen to be delivered, or the oxygenbacklog. When a patient finally does inhale, the flow of oxygen will beproportionally increased based on the oxygen backlog. If the patient'sbreaths are weak, then a high flow of oxygen would cause the pressure tobe positive, and oxygen would stop being delivered, thus reducing wastedoxygen.

While the invention has been described in particular with reference tocertain illustrated configurations, such is not intended to limit thescope of the invention. The present invention may be embodied in otherspecific forms without departing from its spirit or essentialcharacteristics. The described configurations are to be considered asillustrative and not restrictive. The scope of the invention is,therefore, indicated by the appended claims rather than by the foregoingdescription. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

1. An apparatus to deliver a variable flow of oxygen to a patient, theapparatus comprising: a flow control valve to deliver a flow of oxygenthrough an oxygen delivery device; a pressure sensor to detect thepatient's breathing pressure and provide a plurality of breathingpressure values, and to measure ambient pressure and provide an ambientpressure value, the pressure sensor in communication with a processor;the processor programmed to receive the plurality of breathing pressurevalues and the ambient pressure value; the processor programmed toanalyze the plurality of breathing pressure values to determine if apatient is inhaling or not inhaling, and the processor furtherprogrammed to calculate an amount of oxygen to deliver to a patientduring a predetermined time frame, the amount of oxygen to deliver tothe patient being calculated by a pre-selected oxygen flow rate plus anoxygen backlog; and the processor programmed to open the flow controlvalve when the processor determines the patient is inhaling and theoxygen backlog is greater than zero.
 2. The apparatus of claim 1,further including an oxygen flow analyzer to measure a parameterindicative of oxygen flow rate through the oxygen delivery device andprovide an oxygen flow rate value, the processor programmed to receivethe oxygen flow rate value.
 3. The apparatus of claim 1, wherein theprocessor is programmed to increment the oxygen backlog during thepredetermined time frame by the pre-selected oxygen flow rate multipliedby the predetermined time frame.
 4. The apparatus of claim 2, whereinthe processor is further programmed to decrement the oxygen backlogduring the predetermined time frame when the processor determines thepatient is inhaling.
 5. The apparatus of claim 4, wherein the processordecrements the oxygen backlog by the oxygen flow rate value multipliedby the breathing pressure value.
 6. The apparatus of claim 1, whereinthe flow control valve includes a proportional valve having a solenoid,and wherein the processor is programmed to send a current input to thesolenoid when the processor determines the patient is breathing.
 7. Theapparatus of claim 6, wherein the processor calculates the current inputto send to the solenoid by comparing the breathing pressure value to theoxygen backlog, and sending the current input based on the smaller ofthe breathing pressure value and the oxygen backlog.
 8. The apparatus ofclaim 1, wherein the pressure sensor comprises a patient breathingpressure sensor to detect the patient's breathing pressure and provide aplurality of breathing pressure values, and an ambient pressure sensorto measure ambient pressure and provide an ambient pressure value, thepatient breathing pressure sensor and the ambient pressure sensor incommunication with the processor.
 9. The apparatus of claim 2, whereinthe oxygen flow analyzer comprises a heated-wire type anemometer. 10.The apparatus of claim 2, wherein the oxygen flow analyzer comprises adifferential-type pressure analyzer.
 11. A method for delivering avariable flow of oxygen to a patient, the method comprising: detecting apatient's breathing pressure to determine a breathing pressure value;receiving the breathing pressure value at at least one processor; the atleast one processor incrementing the total amount of oxygen to bedelivered based on a pre-selected oxygen flow rate and a predeterminedtime interval; the at least one processor analyzing the breathingpressure value to determine if the patient is inhaling or exhaling, andwhen the processor determines the patient is inhaling, the at least oneprocessor calculating an optimal oxygen flow rate based on the totalamount of oxygen to be delivered and the breathing pressure value; theat least one processor sending a signal to a flow control valve to openthe valve sufficiently to deliver the optimal oxygen flow rate;measuring an oxygen flow rate through an oxygen delivery device toprovide an oxygen flow rate value; receiving the oxygen flow rate valueat the least one processor; and the at least one processor decrementingthe total amount of oxygen to be delivered based on the oxygen flow ratevalue.
 12. The method according to claim 11, where the step of the atleast one processor incrementing the total amount of oxygen to bedelivered comprises the at least one processor multiplying thepre-selected oxygen flow rate by the predetermined time interval. 13.The method according to claim 11, where the step of the at least oneprocessor analyzing the breathing pressure value to determine if thepatient is inhaling or exhaling comprises the at least one processoranalyzing the breathing pressure value to determine if it is negative ornot negative, and the at least one processor assuming a negativebreathing pressure value is due to an inhalation.
 14. The methodaccording to claim 11, wherein the step of the at least one processordecrementing the total amount of oxygen to be delivered based on theoxygen flow rate value includes the at least one processor multiplyingthe oxygen flow rate value by the predetermined time interval tocalculate a calculated volume of oxygen delivered, and the at least oneprocessor decrementing the total amount of oxygen to be delivered by thecalculated volume of oxygen delivered.
 15. The method according to claim11, wherein the flow control valve comprises a proportional valve havinga solenoid, and wherein step of the at least one processor sending asignal to a flow control valve to open the valve sufficiently comprisesthe at least one processor sending a current input to the solenoid. 16.A method for delivering a variable flow of oxygen to a patient, themethod comprising: detecting a patient's breathing pressure to determinea breathing pressure value; receiving the breathing pressure value at atleast one processor; the at least one processor analyzing the breathingpressure value to determine if the patient is inhaling or exhaling, andwhen the processor determines the patient is inhaling, the at least oneprocessor calculating an optimal oxygen flow rate based on the breathingpressure value; and the at least one processor sending a signal to aflow control valve to open the flow control valve sufficiently todeliver the optimal oxygen flow rate.
 17. The method according to claim16, wherein the step of calculating an optimal oxygen flow rate based onthe breathing pressure value comprises calculating the optimal oxygenflow rate as equal to the negative value of the breathing pressurevalue.
 18. The method according to claim 16, wherein the flow controlvalve comprises a proportional valve having a solenoid, and wherein stepof the at least one processor sending a signal to the flow control valvecomprises the at least one processor sending a current input to thesolenoid.